This invention relates to the use of the Hall Effect in electronic devices to change the resistance of electronic components for a variety of applications, for example, to store information, to program logic devices, to sense or measure magnetic fields and derived quantities, as well as other applications.
The Hall Effect is a well known physical effect sometimes used in scientific equipment and for magnetic field measurements. The Hall Effect is the creation of an electric voltage inside an electrical conductor when it is conducting an electric current in the presence of a magnetic field. This magnetic field pushes the electronic carriers to one side of the conductor, giving rise to the Hall voltage. The Hall voltage is proportional to the applied magnetic field and to the electric current. It can be measured and used to determine other parameters. The Hall voltage is on the order of millivolts or microvolts for practical values of electric current and magnetic field. In most practical applications, however, the Hall Effect is insignificant. To a first order approximation the electrical resistance of a conductor between two points in a conductor, for example, points A and B in FIG. 1, is unaffected by the magnetic field and by the induced Hall voltage. The reason for this Hall independent behavior is that the total electrical conductance depends on the total sum of the carriers, not on their distribution in the conductor. A further description of the Hall Effect is found in Putley, The Hall Effect and Semiconductor Physics, Dover Publications, Inc., New York, 1968.
The two most prevalent types of semiconductor memory--dynamic random access memory (DRAM) and static random access memory (SRAM)--suffer from an inherent drawback. Each loses its stored information when electric power is disconnected. Read only memories (ROM) do not lose the stored information with the loss of electric power; however, information can be stored only once. ROM's are not practical in any application where their content is to be modified. This includes virtually all the memory requirement of electronic computing equipment. Other types of semiconductor devices--fusible-link devices and ultraviolet erasable programmable read only memories (UVPROM) push the time of programming beyond the device fabrication process. Programming occurs after the integrated circuits have been manufactured and packaged.
Recently, a type of memory known as electronically erasable read only memory (EEPROM) has become practical. EEPROM does not lose information when power is disconnected. Practical EEPROMs, however, suffer from a number of shortcomings. They are slow to program, and the data survive only a limited number of read/write cycles. Typically, large blocks of data are erased at one time (known as "flash memory"). These devices cannot be used for computer main memory--or for any application which requires fast, frequent loading and storing of data. No devices exist at present, which are both nonvolatile and practical as main memory for computers, both for storage of data and code.
Large, reliable, and inexpensive (but relatively slow) nonvolatile memory is available in the form of magnetic disks. In a typical hard disk drive a flat disk is coated with a magnetic thin film. Portions of the magnetic thin film are oriented in specific directions. The orientation of these magnetic domains identifies the stored information as either a "one" or a "zero" bit. To read or write data the magnetic disk rotates at high speed (typically 3600 rpm or higher), and a "magnetic head" is located in close proximity to the surface. During reading the head senses the contents of the magnetic domains as they move underneath it. It is also used to write areas of the disk during data storage.
For writing a bit, current is applied to a wired loop on the head, and to a structure which causes its magnetic field to penetrate into the magnetic thin film on the surface of the disk and magnetizes the area under the header appropriately to write a one or zero. To read the stored content of the thin film, its magnetic field is sensed by the head and translated into a voltage pulse whose polarity corresponds to the orientation of the underlying magnetic domain. The sensing is typically achieved by a conducting loop on the head, a magnetoresistive element, or a traditional Hall voltage sensor. The generated signal is quite small (millivolt range), and susceptible to noise. Complex circuitry is required to amplify and filter the sensed signal for further use. Also, magnetic disks are fragile mechanical devices, sensitive to failure under mechanical shock. This obstructs their use in many portable applications.
The achievable information storage density of hard disk drives depends significantly on the sensitivity of the magnetic field detection by the head. More sensitive detection directly translates into a higher hard disk storage density.
While magnetic disk technology is well established, access to the stored information is too slow to be useful as computer main memory. This is primarily because of the mechanical limitations of the disk rotation and moving the head to the addressed disk area. Also, accesses to the stored information are fundamentally sequential, and not random access.
Programmable logic devices allow changing the logic functionality of the device itself by modifying connections and/or current paths inside the device itself. Virtually all technologies useful for storing information in electronic devices by changing resistances or by opening/closing electronic switches or transistor paths can be used to alter logic inside the programmable logic devices. This includes ROM technology, UV ROM technology and EEROM technology, with all the previously described shortcomings of speed, field programmability, and the number of programming cycles, respectively.